Ally Kazmucha, writing for iMore, on an undisclosed-by-Apple aspect of Touch ID:
Apple doesn't make specific mention of any other additional safeguards. However, as we discovered through some research and testing of our own, it seems as though Apple went through the trouble of pairing every individual Touch ID sensor cable to each individual phone as well. That's an incredible feat, and it immediately raises the question - why?
So the iPhone 5S in my pocket has not only a unique A7 SoC, but also a unique Touch ID sensor.
Apple, sweating the details.